The present invention relates to a composition of an agent and a method for the treatment of molten ferrous metal, and more particularly to a magnesium-containing agent and method for the treatment of molten ferrous metal.
Cast iron is primarily an iron alloy that contains carbon and silicon. Wide variations of the properties of cast iron material can be achieved by varying the amount of carbon and silicon, and by adding various metallic alloys to the cast iron. Commercial cast irons include gray, ductile, malleable, compacted graphite and white cast iron, among others. With the exception of white cast iron, the cast iron steels have a common micro-structure that consists of a graphite phase and a matrix that may be ferritic, pearlitic, bainitic, tempered martensitic, or combinations thereof. Gray iron has flake shaped graphite, ductile iron has nodular or spherically shaped graphite, compacted graphite iron (also called vermicular graphite iron) is intermediate between these two, and malleable iron has irregularly shaped globular graphite that is formed during tempering of white cast iron. White cast irons do not have any graphite in the microstructures, but instead the carbon is present in the form of cementite (iron carbide). Cast irons are also classified as either unalloyed cast irons or alloyed cast irons. Unalloyed cast irons are essentially iron-carbon-silicon alloys with only trace amounts of other elements. Alloyed cast irons are considered to be those casting alloys based upon iron-carbon-silicon systems that contain one or more alloying elements that are intentionally added to enhance one or more useful properties of the cast iron.
In the production of ductile and compacted graphite cast iron, pure magnesium or alloys having varying amounts of magnesium are added to molten cast iron. The addition of the magnesium to the cast iron improves the strength properties of the cast iron by modifying the graphite matrix in the cast iron. Various techniques can be used to introduce the magnesium into the cast iron. Small particles of pure magnesium can be directly added to the molten cast iron. The magnesium particles can be plunged into a ladle of the molten cast iron. Injection of the magnesium particles through a lance can be used, but this method requires large volumes of transport gas, otherwise the magnesium particles melt prior to being injected into the molten cast iron thus resulting in the plugging of the lance. The large volumes of transport gas can cause severe splashing, rendering the process impractical. The addition of magnesium particles on the surface of the molten cast iron is generally not used since much of the magnesium vaporizes before it can modify the cast iron. Magnesium has a boiling point of about 2025xc2x0 F. The cast iron in the ladle or melting pot is generally maintained at about 2300-2850xc2x0 F. As a result, the magnesium rapidly vaporizes on contact with the molten cast iron and vaporizes into a gas without modifying the cast iron. Several methods have been developed to increase the recovery of the magnesium on the cast iron. For example, one method involves magnesium deposited on the bottom of the melting pot or ladle and then being covered with reaction retarding steel plates, whereupon the iron is poured over the magnesium. Other methods require similar cumbersome preparation.
The most common method for producing ductile and compacted graphite cast iron alloys is to add ferrous metal alloys that include magnesium into the molten cast iron. The ferrous metal alloys typically are made of iron, silicon and magnesium so as to not introduce any undesired substances into the cast iron. The ferrous metal alloy is introduced in solid form into the molten cast iron. The ferrous metal alloy slowly melts in the molten cast iron and the magnesium in the ferrous metal alloy is recovered in much higher percentages than compared with adding pure magnesium to the cast iron.
The ferrous metal alloy is commonly made by smelting liquid ferro-silicon alloys in dedicated furnaces and then tapping the liquid ferro-silicon alloys in transport ladles and adding metallic magnesium in the form of large ingots in the liquid bath in an amount sufficient to obtain the desired magnesium content in the ferro-silicon alloy. Another common method used to add magnesium to the ferro-silicon alloy is to add the metallic magnesium in the form of cored wire with the metallic magnesium contained in a rod formed by a steel sheath. In each of these production methods, the liquid bath and the transport ladle must be stirred, by mechanically stirring the bath with the addition, and/or by stirring with inert gas injected through a porous plug within the ladle and/or through a lance submerged into liquid bath. After the desired amount of magnesium is obtained in the ferro-silicon alloy, the liquid ferro-silicon is poured out of the ladle for solidification for further use by the gray iron foundries. Another method used to add magnesium to molten ferro-silicon alloy is the injection of magnesium granules through a refractory lance. Besides delivering the magnesium directly to the bottom of the bath, at the end of the injection lance, the injection method enables the user to add other alloy fines as a blend with the magnesium granules. However, experience with injection of magnesium into molten pig iron in the steel industry has shown that unless large quantities of transport gas are used, magnesium particles injected alone, without any carrier material, will tend to melt inside the lance, thus plugging the transport pipe, resulting in much lost time and expense in the unplugging of the lance. Unfortunately, the carrier materials used for the injection of magnesium into molten pig iron, for example lime and/or calcium carbide, can also introduce unwanted contaminants into certain grades of ferro-silicon alloys.
In view of the present methods for the formation of magnesium-ferro-silicon alloys for the subsequent use in the alloying of cast iron, there is a need for an improved method and additive for the formation of magnesium-ferro-silicon alloys which results in increased amounts of magnesium alloying and which simplifies the alloying process and reduces the costs and wastes associated with the formation of the magnesium-ferro-silicon alloys. Moreover, these treatment agents and methods used for introducing magnesium into the molten ferrous metal, ferro-silicon, can also be applied for the treatment with magnesium of the molten ferrous metal, cast iron, for the production of ductile cast iron.
The present invention overcomes the problem with adding magnesium particles by the injection of magnesium particles alone into the ferro-silicon alloys by using an improved mixture of treatment particles. The present invention also simplifies the alloying process, eliminates the need for adding possible contaminates to the magnesium-ferro-silicon alloy, improves the amount of alloying of the magnesium in the ferro-silicon alloy, and/or reduces the amount of waste associated with the production of the magnesium-ferro-silicon alloy. However, the invention has broader applications in that the treatment particles can be directly added to molten iron to alloy and/or desulfurize the molten iron without the use, or in combination with the use, of a magnesium-ferro-silicon alloy.
In accordance with the principal aspect of the present invention, magnesium particles are injected into a ferro-silicon alloy by a lance to alloy a desired amount of magnesium in the ferro-silicon alloy. The melting of the metallic magnesium in the transport pipe of the lance is inhibited or overcome by mixing the magnesium particles with high melting temperature particles. The high melting temperature particles are designed to absorb heat as the high melting temperature particles and the magnesium particles are transported through the lance and into the ferro-silicon alloy. The absorption of heat by the high melting temperature particles inhibits or prevents the magnesium particles from melting or completely melting prior to being injected into the molten ferro-silicon alloy. By inhibiting the melting of the magnesium particles in the lance, the problems associated with plugging of the lance during the magnesium alloying of the molten ferro-silicon alloy is overcome. In one embodiment, the magnesium particles are made of a majority of magnesium. In one aspect of this embodiment, the magnesium particles are made up of over 90% magnesium, preferably over 95% magnesium, and even more preferably over 98% magnesium. In another embodiment, the high melting temperature alloy particles are made up of two or more of the following metals, namely, aluminum, antimony, beryllium, boron, calcium, chromium, copper, iron, magnesium, manganese, nickel, rare earth metals, silicon, silver, sodium, strontium, tin, titanium, vanadium, zinc, zirconium, and mixtures thereof. In one aspect of this embodiment, the high melting temperature particles include iron and silicon. In another aspect of this embodiment, the high melting temperature particles include iron, magnesium and silicon. The specific composition of the high melting temperature particles is selected to obtain the desired heat absorbing characteristics of the particles when used in combination with the magnesium particles. The specific composition of the high melting temperature particles is also preferably selected to minimize contamination of the molten ferro-silicon alloy. As can be appreciated, if the final composition of the ferrous metal should not include aluminum, the high melting temperature particle should not include aluminum so as not to introduce aluminum into the ferro-silicon alloy which in turn is later added to the molten ferrous metal. In still another embodiment, the high melting temperature particles include iron, silicon and magnesium or iron and silicon to avoid contamination of the molten ferrous material by unwanted elements. The use of magnesium-ferro-silicon alloy or ferro-silicon alloy as the high melting temperature particle simply adds more material of similar composition to the molten ferro-silicon, thus not contaminating the ferro-silicon alloy with undesired elements. Materials commonly used as a carrier for metallic magnesium in other applications, such as hot metal desulfurization, which include lime or calcium carbide, can introduce calcium to the magnesium-ferro-silicon alloy, which is unwanted for certain grades of alloy. The present invention avoids the addition of unwanted elements. However, certain grades of ferro-silicon require a minimum calcium content. For these grades, the use of lime or calcium carbide as the material for the high melting temperature particles would be very appropriate. In this embodiment, magnesium particles and lime and/or calcium carbide particles are injected into the molten ferro-silicon bath, with the aim of recovering both magnesium and calcium from the injected particles.
In accordance with a further embodiment of the invention, the magnesium particles and high melting temperature particles are added to a ferrous alloy. In one aspect of this embodiment, the ferrous alloy is substantially iron. In another aspect of this embodiment, the ferrous alloy is a ferro-silicon alloy. Preferably, the ferro-silicon alloy includes 15-95% silicon and 5-85% iron. In accordance with still a further embodiment of the invention, the magnesium particles are added in a sufficient quantity to the ferrous alloy such that about 0.5-20% magnesium is alloyed in the ferrous alloy.
In accordance with another aspect of the present invention, the ratio of high melting temperature particles to the magnesium particles is selected so as to ensure that the magnesium particles do not sufficiently melt in the lance to cause clogging of the lance during the injection of the magnesium particles and high melting temperature particles into the ferro-silicon alloy. In one embodiment, the amount of magnesium particles in the particle mixture ranges from about 5% to 90% of the mixture, and preferably 60% to 90% of the mixture. The ratio of the metallic magnesium to the high melting temperature particles varies depending on the type of molten alloy, e.g. ferro-silicon alloy, desired and the composition of the high melting temperature particles.
In yet another aspect of the present invention, an injection lance is used to inject magnesium into the molten ferrous alloy (e.g. ferro-silicon alloy) and to improve the alloying of the magnesium in the ferrous alloy. When the magnesium particles are stirred into a liquid bath of the molten ferrous alloy, significant amounts of magnesium are lost by vaporization as fumes and oxidation as white smoke when the magnesium melts in the molten ferrous alloy. The injection of magnesium particles through an injection lance immerses the magnesium particles in the bath to minimize the oxidation of the magnesium and to reduce the vaporization of the magnesium prior to alloying with the ferrous alloy, thus allowing the magnesium to dissolve more completely in the molten ferrous alloy before it reaches the surface of the bath. Furthermore, the reduced loss of magnesium results in increasing economic benefits for the process. The conveying gases of the particles also assist in stirring the particles in the molten ferrous alloy. As a result, the need of a stirring device can be eliminated.
In still another aspect of the present invention, fines which are generated during the casting process of the ferro-silicon, magnesium-ferro-silicon alloy, or the like, can be recycled and used as part of the high melting alloy particles in a subsequent magnesium alloying process. By being able to recycle and remelt these metallic fines from past casting processes, increased recovery of the metallic content of the fines is obtained, resulting in increased economic benefits of the alloying process and less waste. In accordance with still another aspect of the present invention, the composition of the high melting temperature particles is selected to have a melting point which is sufficiently high such that when such particles are combined with the magnesium particles and injected through the lance, the high melting temperature particles absorb a sufficient amount of heat to prevent or inhibit the complete melting of the magnesium particles in the lance. In one embodiment, the average melting temperature of the high melting temperature particles is about 2200xc2x0 F.
In accordance with still yet another aspect of the present invention, the magnesium particles and the high melting temperature particles are injected into the molten ferrous alloy (i.e. ferro-silicon alloy) as a pre-blended mixture. In one embodiment, the magnesium particles and the high melting temperature particles are at least partially mixed prior to injecting the particles into the lance. In one aspect of this embodiment, the magnesium particles and the high melting temperature particles are substantially mixed prior to injection into the lance. In another embodiment, the magnesium particles and the high melting temperature particles are co-injected into a lance from separate dispensers and the particles are mixed in the lance prior to being conveyed into the molten ferrous alloy. In this aspect of the embodiment, the magnesium particles are blended with at least 10% high melting temperature alloy fines (e.g. ferro-silicon, magnesium-ferro-silicon, etc.) to reduce the chance of inadvertent combustion of the magnesium particles during handling and transport. In another embodiment of the invention, the method of injection through a lance consists of injecting through the single transport line, or from a second set of injectors through a second transport line containing the same lance, i.e. using a dual port lance. Preferably, the particles are fluidized as a suspension of particles in a carrier gas before being injected into the lance. The particle size of the magnesium particles and the high-melting alloy particles is generally the same; however, they can be different. Preferably, the particles are coated with a flow treatment agent such as glycol or a compound of silicon to enhance their fluidization during transport to the lance. The fluidized particles can be carried through the lance by a carrier gas. The carrier gas is preferably inert. The carrier gases commonly used are argon, nitrogen, helium, natural gas, or various other non-oxidizing gases. Preferably, the carrier gas is nitrogen. Generally, the pressure of the carrier gas necessary to inject the particles into the molten ferrous material is about 20-90 psig; however, the pressure may be more or less depending on the particle size of the particles and the depth in which the particles are injected into the molten ferrous alloy. The injection of the magnesium particles into the molten ferrous material not only increases the alloying of the magnesium in the molten ferrous material, the transport gases also increase the mixing of the particles in the molten ferrous alloy to facilitate in the even alloying and distribution of the particles in the molten ferrous alloy.
In accordance with still a further aspect of the present invention, the particles of magnesium and high melting temperature particles can be adapted for use in gray iron foundries. These foundries produce nodular cast iron in a process known as inoculation by introducing magnesium into the cast iron.
An object of the present invention is to provide a new alloy mixture and method of combining the alloying mixture with a molten ferrous material to alloy magnesium with the molten ferrous material.
Another object of the present invention is to provide an alloying mixture which includes a plurality of different particles.
Yet another object of the present invention is to provide an alloying material which includes high melting temperature particles to inhibit or prevent the melting of magnesium particles prior to particles being combined with the molten ferrous material.
In still yet another object of the present invention is to provide an alloying mixture which can be inserted into a molten ferrous material by injection.
A further object of the present invention is to mix magnesium particles with high melting temperature particles prior to injecting the particle mixture into molten ferrous material.
Another object of the present invention is to use a lance or co-injection lance to inject metal alloying particles into molten ferrous material.
In still another object of the present invention is to improve the alloying of magnesium metal in molten ferrous material.
It is still yet another object of the present invention to reduce the loss of magnesium by vaporization or oxidation during the alloying process.
A further object of the present invention is to provide an alloying mixture which can include metal fines from a previous casting process so as to improve metal recovery and/or improve the economics of the process.
These and other objects of the present invention will become apparent to one skilled in the art upon reading the detailed description of the invention in combination with the drawings.